Metal coatings are applied onto surfaces of ceramic bodies for a variety of purposes and in a variety of ways. Generally in all such cases, firm adherence of the coatings often is made possible by a multitude of very small depressions or indentations or pores in such surfaces. Integral parts of the coatings extend into the depressions usually to effect strong durable mechanical keying or bonding of the coatings to the surface. These depressions or indentations may be either substantial customary porosity of the bodies opening onto the surfaces thereof to be coated or the result of roughening the surfaces to be coated by grit blasting or by chemical treatment like etching or leaching (of U.S. Pat. Nos. 2,195,436, 2,872,312, 3,083,109, 3,136,658, 3,296,012, 3,598,635, 3,690,921 and 3,941, 673). Concerning specific chemical treatments:
U.S. Pat. No. 2,195,436 suggests including sodium fluoride in a coating solution of a thermally decomposable metal compound applied to a ceramic so that the fluoride preliminarily reacts with and roughens the surface of the ceramic such as glass, porcelain, fused silica and alundum (alumina).
U.S. Pat. No. 2,872,312 recites the use of a sulphurichydrofluoric acid solution to roughen by etching the surface of a ceramic, e.g. aluminum oxide, so as to form minute surface irregularities or pores thereon.
U.S. Pat. No. 3,296,012 describes the provision of a submicroscopically porous surface on a ceramic by leaching with a suitable boiling acid, such as orthophosphoric acid for alumina and hydrofluoric acid for zircon, beryllia, steatite and silicate glass-ceramics.
U.S. Pat. No. 3,690,921 discloses molten alkali metal hydroxides as etchant for altering the surface of ceramics such as alumina, zircon, beryllia, steatite and silicate glass-ceramics.
For the purpose of degreasing calcia- or yttria-stabilized zirconia (but apparently not roughening it) so that a platinum coating will more readily adhere thereto, U.S. Pat. No. 3,433,680 mentions washing that ceramic with an organic solvent and/or aqua regia.
In attempts to roughen the smooth surfaces of impervious zirconia-base ceramics to facilitate firm adherence of metal coatings thereon, none of the reactants, etchants, leachants or acid degreaser disclosed in the prior art noted above were found to roughen those surfaces so as to effect such firm adherence. While grit blasting of the zirconia-base ceramics did roughen their surfaces to facilitate adherence of metal coating, those blasted ceramics were so mechanically weakened that they broke up when subjected to elevated temperature service conditions. These zirconia-base ceramics contain more than 50 wt. % ZrO.sub.2. Many include one or more of the stabilizer oxides of magnesium, calcium, yttrium, rare earth metal, etc., which yield partially or wholly stabilized zirconia (i.e. partially or wholly composed of thermally stable cubic and/or tetragonal crystal phase), depending upon the amount of stabilizer oxide and firing temperature as is known. These impervious or gas-tight zirconia-base ceramics with smooth surfaces are fabricated from fine grained particulate raw material that is molded into shaped bodies which are fired to develop well sintered structures, e.g. as illustrated in U.S. Pat. Nos. 3,287,143 and 3,365,317.